The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In an HCCI (Homogeneous Charge Compression Ignition) engine, combustion is flameless, and occurs spontaneously occurs throughout the entire combustion chamber volume. The homogeneously mixed cylinder charge is auto-ignited as the cylinder charge is compressed and its temperature increases. The ignition timing of auto-ignited combustion depends on initial cylinder charge conditions including primarily temperature, pressure, and composition. Thus, it is important to coordinate engine control inputs, such as fuel mass, injection timing, and intake and exhaust valve motion, to ensure robust HCCI combustion. Depending on the motion of the intake and exhaust valves, there are two primary operating strategies in HCCI engines—an exhaust recompression strategy and an exhaust re-breathing strategy.
One of the major concerns in an application of an HCCI engine is that auto-ignited combustion is vulnerable to variations in fuel octane levels and variations in environmental conditions including ambient temperature and humidity. This is because such variations can change the cylinder charge conditions and, as a result, significantly affect the ignition timing of auto-ignited combustion, and may induce unwanted vibrations in engine output. Thus, it is important to coordinate the engine inputs/actuators in real time such that robust HCCI combustion can be maintained in the presence of such variations.
Referring now to FIG. 1, a data graph of cylinder charge temperature at intake valve closing (IVC) versus exhaust gas temperature (EXH) demonstrates typical steady state operating characteristics of combustion phasing of auto-ignited combustion of a typical HCCI engine, wherein differences in the ambient environment and the fuel octane are compared. The results are plotted for temperatures TIVC and TEXH, wherein TIVC comprises temperature of an intake charge at intake valve closing and TEXH comprises the exhaust gas temperature. The dotted lines comprise lines of TIVC and TEXH required for constant combustion phasing, defined herein as a crank angle at which 50% of the inducted mass fuel is burned (CA50). The results indicate that the cylinder charge temperature varies with the amount of hot residual gas (i.e., internal EGR) trapped in the cylinder. One of the reasons that the combustion phasing is retarded in a high temperature zone is because the mixture eventually burns progressively slower due to a higher dilution level as the amount of hot residual gas trapped in the cylinder increases. The qualitative effect on combustion due to ambient environment and/or fuel octane changes can be seen by comparing Lines 102 and 104. For example, it is shown that combustion phasing retards with either an increase in fuel octane number or an increase in humidity. Referring now to FIG. 2, there is depicted experimental data from an exemplary HCCI engine operating with different types of fuel, which have varying octane numbers. The engine was operated at a fixed engine speed of about 1000 RPM, with a fixed fuel injection flowrate. The results indicate that combustion phasing retards as the octane number of the fuel increases from a low octane rating, to a medium octane rating, to a high octane rating. The octane rating comprises a standard fuel rating system used to describe or define an anti-knock quality, i.e., resistance to uncontrolled pre-ignition, of fuel intended for use in spark-ignition and HCCI engines. Resistance to uncontrolled pre-ignition increases with increased octane rating. The results depicted in FIG. 2 indicate that the characteristics of combustion phasing do not change qualitatively with environmental and/or fuel octane changes.
Referring now to FIG. 3, there are depicted steady state characteristics of combustion phasing of auto-ignited combustions with external EGR mass flow. Lines 302, 304, and 306 represent lines of constant combustion phasing, indicated by 50% of fuel mass burn-point (CA50), for crank angles of 0, 10, and 20 degrees after combustion top-dead-center (aTDC), respectively. Lines 312 and 314 represent relations between TEXH and TIVC with two different sets of constant valve timing and profile, and the valve timing and profile used in the line 312 introduces a lower internal EGR than those used in line 314. Line 308 represents HCCI engine operation without external EGR, and 310 represents HCCI engine operation with 5% external EGR. The results indicate that combustion phasing can be controlled by adjusting either internal or external EGR mass flow, or both. Typically, HCCI engines employ a mechanism to control valve timing/profile to adjust the internal EGR, e.g., cam phasing or a flexible valve control. The exhaust gas from the previous engine cycle is trapped and increases the charge temperature to a point sufficiently hot enough for auto-ignited combustion. The amount of the exhaust gas trapped in the cylinder determines charge temperature, and thus, the combustion phasing of auto-ignited combustion. However, as demonstrated by the results depicted in FIG. 3, combustion phasing does not monotonically change with internal EGR for all conditions. Thus, combustion phasing control using internal EGR is typically limited to operating conditions whereat the combustion phasing changes monotonically with internal EGR. Such operating conditions vary with environmental and fuel quality changes, which can change during real time operation. Without precise information about environmental and fuel quality changes, it is challenging to identify changes in operating conditions in real time.
The results depicted in FIG. 3 indicate that increasing external EGR mass flow monotonically increases temperature of exhaust gases for any given intake charge temperature. Therefore, combustion phasing can be controlled by adjusting and controlling external EGR flow mass. This monotonic relation exists between combustion phasing and external EGR regardless of ambient environment and/or fuel octane variations. Thus, changes in the external EGR can be used to control combustion phasing of auto-ignited combustion in response to changes in ambient environment and/or fuel octane. The figures demonstrate that although combustion phasing shifts with changes in one of the ambient environment the fuel octane, the qualitative behavior of combustion over the cylinder charge temperature remains the same.
The invention described hereinafter comprises a method and a control scheme to control operation of an HCCI engine to compensate for variations in ambient conditions and engine fuel octane, especially fuel octane level.